The present invention relates to tubular food casings from cellulose films and more particularly relates to tubular food casings formed by extrusion of a solution of cellulose-followed by precipitation of the cellulose to form a tubular cellulose film.
In order to obtain a film that is strong enough and tough enough to be used as a food casing, e.g. for sausage casings, it has traditionally been believed that the cellulose had to have a relatively high molecular weight, e.g. as represented by its degree of polymerization (DP). The degree of polymerization that was believed to be required for a food casing of sufficient strength and toughness for commercial use was at least 560.
In the prior art, in order to dissolve cellulose, it was almost always first treated with sodium hydroxide to reduce the strength of hydrogen bonds and to expand it to permit the solvent to work more easily. Cellulose of sufficient DP to make a food casing, having good enough physical properties to be practical, still could not be dissolved to any significant degree in sodium hydroxide solution alone. However, there are no practical solvents for cellulose that function alone and such practical solvents, as do exist, usually require an alkali metal hydroxide as a cosolvent. Cellulose, for example, will not dissolve in aqueous carbon disulfide, or tertiary amine oxide, to any significant extent unless the cellulose is first expanded (steeped) in sodium hydroxide and the solution itself contains alkali metal hydroxide, preferably sodium hydroxide.
The invention is a tubular food casing of a tubular cellulose film precipitated from a viscose solution having a viscosity of from about 55 to about 90 ball seconds, where the ball has a density of 8 g/cc and a radius of 0.316 cm, and where the solution contains at least eight and one-half weight percent of cellulose, said cellulose having a DPv of from about 300 to about 525. The cellulose film has a dry film thickness of from about 0.015 mm to about 0.040 mm, a dry burst pressure in excess of 40 cm Hg, per 0.01 mm of dry film thickness, and a rewet burst pressure in excess of 5 cm Hg per 0.01 mm of rewet film thickness.
The cellulose may be precipitated from solution of non-derivatized cellulose, e.g. from aqueous tertiary amine oxide solution or may be regenerated from a solution of derivatized cellulose, e.g. a solution of cellulose xanthate.
The invention also includes a method for making the cellulose film by:
a) preparing a viscose solution, containing at least eight and one-half weight percent of cellulose having a DPv of about 300 to about 525, and having a solution viscosity of from about 55 to about 90 ball seconds, where the ball has a density of 8 g/cc and a radius of 0.316 cm.
b) extruding the solution into the shape of a tube; and
c) precipitating cellulose from the extruded solution to form a tubular film having a dry film thickness of from about 0.015 mm to about 0.040 mm, a dry burst pressure in excess of 40 cm Hg, per 0.01 mm of dry film thickness, and a rewet burst pressure in excess of 5 cm Hg per 0.01 mm of rewet film thickness.
The cellulose used in accordance with the invention has a low DPv, e.g. from about 300 to about 525 and usually from about 400 to about 475. The viscose (xanthate or traditional viscose) may be a derivatized cellulose, e.g. xanthanated with carbon disulfide, dissolved in caustic at a concentration of from about 4.5 to about 6.5 weight percent. The viscose total sulfur concentration is usually from about 1.8 to about 2.5 weight percent and to form a cellulose film, the cellulose is precipitated and regenerated from the xanthate by passing extruded viscose through a bath comprising a strong acid and a salt. The viscose may also be a solution comprising non-derivatized cellulose in a solvent comprising tertiary amine oxide and water (amine oxide viscose) obtained by forming a dilute solution of about 300 to about 525 DPv, preferably about 400 to about 475 DPv, cellulose and removing water by vaporization. The cellulose is precipitated by extruding the viscose and passing the extruded viscose through a wash bath containing water to remove tertiary amine oxide.
The viscose may also be a solution of non-derivatized cellulose in aqueous alkali. It has been surprisingly found that solutions of cellulose having low DPv can be obtained by dissolving specially prepared low DPv cellulose in dilute concentration in aqueous alkali followed by removing water, e.g. by vaporization under a partial vacuum, to obtain a cellulose solution in alkali (alkali viscose) having a high cellulose concentration, e.g. in excess of eight weight percent. In such a case, the viscose is obtained by forming a dilute solution of about 300 to about 525 DPv cellulose and removing the water by vaporization where the cellulose is obtained by treating higher DPv cellulose with acid or steam expansion to reduce the DPv.
Tubular cellulose film food casings made in accordance with the present invention have surprisingly good properties when compared with traditional cellulose film food casings made from high DPv cellulose. In particular such films may have a dry film thickness of from about 0.015 mm to about 0.040 mm, a dry burst pressure in excess of 40 cm Hg, per 0.01 mm of dry film thickness, and a rewet burst pressure in excess of 5 cm Hg per 0.01 mm of rewet film thickness.
Tubular food casings of the present invention may also include fiber reinforced films where the viscose is applied to a fiber web, e.g. a fiber paper or where fibers are blended into the viscose. Such tubular food casings are usually thicker and larger than unreinforced tubular film food casings.